Halogen-containing natural products, many of which have interesting biological activities, constitute a major percentage of the secondary metabolites from marine species, but the biosynthetic origins of the halogen atoms is not fully understood. Haloperoxidases, enzymes capable of electrophilic transfer of halogen, are believed to be responsible for most cases of biological halogenation. However, the general distribution and activity of these enzymes among marine species has not been investigated in any detail. The structural diversity of halogenated marine natural products implicates a wide range of activity for haloperoxidases among marine organisms. The objectives of this proposal are to develop a clear understanding of the roles of haloperoxidases in the biosynthesis of halogenated marine secondary metabolites. Haloperoxidases will be isolated and purified from marine plants and animals which are known to produce halogenated natural products. After general haloperoxidase activity is verified by use of standard halonium ion acceptors, such as monochlorodimedone, the halogenating activity of an enzyme will be tested using substrates which may serve as biosynthetic precursors in vivo. Various aspects of haloperoxidase activity will be explored, including their roles in halonium-ion induced cyclization reactions, and in regio- and stereospecific addition of halogen to substrates. Organisms which will be used for these studies will include the acorn worms Saccoglossus kowalewskii and Ptychodera flava, the coral Telesto riisei, and the algae Rhodymedia and Laurencia spp., all of which are known to produce halogenated natural products. The haloperoxidases are related to other heme-containing enzymes such as cytochrome P450 and catalase, as well as enzymes of the human immune system: an understanding of the substrate reactions of haloperoxidase may provide additional information on the biological activities of these enzymes. The haloperoxidases also have potential importance as synthetic tools, such as in production of intermediates for pharmaceuticals, and in the radiolabelling of proteins.